1. Field of the Invention
This invention relates generally to tube support arrangements for steam generators and more particularly to a tube support arrangement for a tube and shell steam generator that imparts a preload on the tubes.
2. Description of Related Art
A pressurized water nuclear reactor steam generator typically comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with a primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside, and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., that is the side of the steam generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators. This mixture, called the downcomer flow is conducted down the annular chamber adjacent the shell until the tube sheet located at the bottom of the annular chamber causes the water to change direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam, and the steam vapor then exits the steam generator and is typically circulated through turbine and electrical generating equipment to generate electricity in a manner well known in the art.
Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect free so that no breaks will occur in the U-tubes that will cause radioactive materials from the primary fluid to enter the secondary side; an undesirable result.
Vibration due to fluidelastic excitation of the heat exchanger tubes can result in wear of the walls of the tubes and breach of the pressure barrier between the primary and secondary fluid systems at the locations where the heat exchanger tubes pass through holes in support plates which are axially spaced along the tube bundle to support the tubes. This is especially a problem in axial flow preheaters, that employ a partition plate to separate the secondary side flow into hot leg and cold leg sides to minimize mixing of the warmer recirculating water with the cooler feedwater. This separation is necessary for the feedwater to be heated on the cold leg side of the unit to increase the unit's heat transfer efficiency. However, due to differences in secondary fluid densities, cross flow occurs at the top of the partition plate, with flow generally streaming from the cold leg side towards the hot leg side. This site has been the location of tube wear in several types of preheat steam generators including axial flow, cross flow and counter flow type steam generators.
Accordingly, it is an object of this invention to control tube bundle vibration to avoid wear of the heat exchanger tubes.
Furthermore, it is an object of this invention to control tube vibration in a manner that does not complicate the loading of the heat exchanger tubes through the support plates and into the tube sheet during manufacture or in a cool, depressurized condition.